Light sources such as LED, according to internal structure or encapsulation form thereof, usually have a certain luminous intensity spatial distribution. For instance, FIG. 2 shows an example of luminous intensity spatial distribution curve (light distribution curve) of an LED chip. As shown in FIG. 2, a luminous intensity value is the biggest in a direction of normal line (direction of 0°) in a light emergent surface of the LED chip, and as emission angle increases, the luminous intensity gradually becomes reduced, and a value in a direction of 90° is the smallest.
FIG. 3A to FIG. 3C show an example of an LED chip useful in a light-emitting device, wherein FIG. 3A is an oblique view of an LED tube 3, FIG. 3B is a cross-sectional view of the LED tube 3 in a direction perpendicular to its length direction, and FIG. 3C is a schematic diagram of light emission of the LED tube 3.
As shown in FIG. 3A and FIG. 3B, the LED tube 3 on the whole in an elongated cylindrical shape and includes a housing 31, a printed circuit board 32 arranged in the housing 31, blue LEDs 33 mounted on the printed circuit board 32 and a phosphor cover 34. The phosphor cover 34 is united with the housing 31, and partially encloses the blue LEDs 33 and is spaced an appropriate distance from the blue LEDs 33. The phosphor cover 34 has a uniform thickness and includes phosphors (not shown) that can convert blue light to yellow light. Accordingly, a part of blue light emitted from the blue LEDs 33 directly passes through the phosphor cover 34 to go outwards, and the other part of blue light is converted to yellow light by phosphors in the phosphor cover 34 and then goes outwards. Finally, the LED tube 3 emits white light obtained by mixing the blue light and the yellow light.
The technology of using a cover body including phosphors is generally called “remote phosphor” technology. In this technology, However, as THE phosphor cover spaced apart from the light source is used, and the light source usually has a certain luminous intensity spatial distribution, as mentioned above, the luminous intensities are different in different light emitting angles or different light emitting directions, the light-emitting device presents color variance indifferent light emitting angles or different light emitting directions. Explanations in conjunction with FIG. 3C are as follows. In FIG. 3C, solid arrows represent blue light, hollow arrows represent yellow light, and lateral width of the arrow schematically represents luminous intensity. As shown in FIG. 3C, the blue LEDs 33 have a luminous intensity in a first direction greater than that in a second direction, and thus, intensity of blue light emitted from the phosphor cover 34 in the first direction is greater than intensity of blue light emitted from the phosphor cover 34 in the second direction. Moreover, as the phosphor cover 34 substantially has the same phosphor concentration and cover thickness in the first direction and in the second direction, intensities of yellow light emitted from the phosphor cover 34 in the first direction and in the second direction are substantially the same. This leads to difference in luminous intensity ratios of blue light to yellow light emitted from the phosphor cover 34 in the first direction and in the second direction. The luminous intensity ratio of yellow light to blue light emitted in the first direction is relatively big, and then the color temperature is quite high, and bluish light on the whole is emitted; while the luminous intensity ratio of blue light to yellow light emitted in the second direction is quite small, and then the color temperature is very low, and yellowish light on the whole is emitted. As a result, there is color variance between light emitted from the LED tube 3 in different directions.
In order to solve this problem, a technical solution of using a phosphor cover with a non-uniform thickness is proposed. Detailed descriptions will be given hereinafter in conjunction with FIG. 4A to FIG. 4D.
FIG. 4A to FIG. 4D show another example of LED chips useful in a light-emitting device, wherein FIG. 4A is a cross-sectional view of an LED tube 4 in a direction perpendicular to its length direction, FIG. 4B is a schematic diagram of light emission of the LED tube 4, FIG. 4C is a schematic diagram of blue light path of the LED tube 4, and FIG. 4D is a schematic diagram of yellow light path of the LED tube 4. In FIG. 4C and FIG. 4D, solid-line arrows represent blue light, and dashed-line arrows represent yellow light. In the following descriptions, only differences from the example as shown in FIG. 3 will be described, and the same or similar parts will be omitted.
As shown in FIG. 4A and FIG. 4B, the LED tube 4 includes a phosphor cover 44 with a non-uniform thickness. The phosphor cover 44 has a big thickness in a first direction where a luminous intensity of the blue LEDs 43 is relatively big, and the phosphor cover 44 has a relatively small thickness in a second direction where a luminous intensity of the blue LEDs 43 is relatively small. By means of such configuration, in the first direction, most of blue light incident on the phosphor cover 44 is converted to yellow light and goes out, and in the second direction, a small part of blue light in the blue light incident on the phosphor cover 44 is converted to yellow light and goes out. Therefore, luminous intensity ratios of blue light to yellow light in light emitted in the first direction and in the second direction are modified, thereby adjusting color variance between light emitted in different directions.
However, this solution again leads to new issues. As shown in FIG. 4C, unconverted blue light penetrates through the phosphor cover 44 and goes outwards, and its propagation basically obeys law of refraction. In a situation that the phosphor cover 44 has a non-uniform thickness, emergent angles of blue light emitted from different points on alight emergent surface of the phosphor cover 44 might be different. And as shown in FIG. 4D, scattering will take place in a process of blue light exciting the phosphors for obtaining yellow light b, and the obtained yellow light b is emergent in multiple directions. Therefore, though mixture ratios of blue light to yellow light emitted in different directions is tried to be adjusted by changing the thickness of the phosphor cover 44, the change in emergent directions of blue light and yellow light due to the change of thickness causes it hard to precisely adjust the mixture ratios of blue light to yellow light emitted in various directions. Hence, that effect of color variance adjustment in this solution is undesirable. Moreover, such color variance cannot be eliminated by further changing the thickness of the phosphor cover 44, because a light path direction is also changed when the thickness is changed to eliminate the color variance so that chroma of light emitted in various directions will be changed again.